The present invention relates to spectrometry, and more particularly, to methodology and apparatus for the analysis of compounds by chromatography-high field asymmetric waveform ion mobility spectrometry.
There is a developing interest in making in situ measurements of chemicals present in complex mixtures at industrial or environmental venues. A fully functional chemical sensor system may incorporate a front end, e.g., a gas chromatography (GC) analyzer as a compound separator, and then a detector, i.e., a spectrometer.
Gas chromatography is a chemical compound separation method in which a discrete gas sample (composed of a mixture of chemical components) is introduced via a shutter arrangement into a GC column. Components of the introduced gas sample are partitioned between two phases: one phase is a stationary bed with a large surface area, and the other is a gas which percolates through the stationary bed. The sample is vaporized and carried by the mobile gas phase (the carrier gas) through the column. Samples partition (equilibrate) into the stationary (liquid) phase, based on their solubilities into the column coating at the given temperature. The components of the sample separate from one another based on their relative vapor pressures and affinities for the stationary bed, this process is called elution.
The heart of the chromatograph is the column; the first ones were metal tubes packed with inert supports on which stationary liquids were coated. Presently, the most popular columns are made of fused silica and are open tubes with capillary dimensions. The stationary liquid phase is coated on the inside surface of the capillary wall.
Compounds are discriminated by the time that they are retained in the GC column (the time from sample injection to the time the peak maximum appears). Chemical species are identified from a sample based on their retention time. The height of any one of these peaks indicates the intensity or concentration of the specific detected compound.
A carrier gas (e.g., helium, filtered air, nitrogen) flows continuously through the injection port, and the column. The flow rate of the carrier gas must be carefully controlled to ensure reproducible retention times and to minimize detector drift and noise. The sample is usually injected (often with a microsyringe) into a heated injection port where it is vaporized and carried into the column, often capillary columns 15 to 30 meters long are used but for fast GC they can be significantly shorter (less than 1 meter), coated on the inside with a thin (e.g., 0.2 micron) film of high boiling liquid (the stationary phase). The sample partitions between the mobile and stationary phases, and is separated into individual components based on relative solubility in the liquid phase and relative vapor pressures. After the column, the carrier gas and sample pass through a detector that typically measures the quantity of the sample, and produces an electrical signal representative thereof.
Certain components of high speed or portable GC analyzers have reached advanced stages of refinement. These include improved columns and injectors, and heaters that achieve precise temperature control of the column. Even so, detectors for portable gas chromatographs still suffer from relatively poor detection limits and sensitivity. In addition, GC analyzers combined with any of the conventional detectorsxe2x80x94flame ionization detectors (FID), thermal conductivity detectors, or photo-ionization detectorsxe2x80x94simply produce a signal indicating the presence of a compound eluted from the GC column. However, presence indication alone is often inadequate, and it is often desirable to obtain additional specific information that can enable unambiguous compound identification.
One approach to unambiguous compound identification employs a combination of instruments capable of providing an orthogonal set of information for each chromatographic peak. (The term orthogonal will be appreciated by those skilled in the art to mean data which enables multiple levels of reliable and accurate identification of a particular species, and uses a different property of the compound for identification.) One such combination of instruments is a GC attached to a mass spectrometer (MS). The mass spectrometer is generally considered one of the most definitive detectors for compound identification, as it generates a fingerprint pattern of fragment ions for each compound eluting from the GC. Use of the mass spectrometer as the detector dramatically increases the value of analytical separation provided by the GC. The combined GC-MS information, in most cases, is sufficient for unambiguous identification of the compound.
Unfortunately, the GC-MS is not well suited for small, low cost, fieldable instruments. Therefore there is still a strong need to be met with a fieldable chemical sensor that can generate reliable orthogonal information. A successful field instrument should include both a small injector/column and a small detector/spectrometer and yet be able to rapidly produce unambiguous orthogonal data for identification of a detected compound.
While GC""s are continuously being miniaturized and reduced in cost, mass spectrometers are still very expensive, easily exceeding $100K. Their size remains relatively large, making them difficult to deploy in the field. Mass spectrometers also suffer from the need to operate at low pressures, and their spectra can be difficult to interpret often requiring a highly trained operator. The search therefore has continued for fieldable spectrometer.
Time-of-flight Ion Mobility Spectrometers (TOF-IMS) have been described as detectors for gas chromatographs from early in the development of ion mobility spectrometry and the first successful use of TOF-IMS detectors with capillary chromatography occurred in 1982. High-speed response and low memory effects were attained and the gas phase ion chemistry inside the TOF-IMS can be highly reproducible providing the foundation to glean chemical class information from mobility spectra. Thus, TOF-IMS, as ionization detectors for GC, do exhibit functional parallels to mass spectrometers, except all processes in IMS occur at ambient pressure making vacuum systems unnecessary. The IMS spectra is also simpler to interpret since it contains fewer peaks, due to less ion fragmentation. The usefulness of a gas chromatograph with TOF-IMS detector has been recognized for air quality monitoring, chemical agent monitoring, explosives detection, and for some environmental uses.
Fieldability still remains a problem for TOF-IMS. Despite advances over the past decade, TOF-IMS drift tubes are still comparatively large and expensive and suffer from losses in detection limits when made small. The search therefore still continues for a successful field instrument that includes both a small ion injector/column and a small detector/spectrometer and yet is able to rapidly produce unambiguous orthogonal data for identification of a detected compound.
The high field asymmetric waveform ion mobility spectrometer (FAIMS) is an alternative to the TOF-IMS. In a FAIMS device, a gas sample that contains a chemical compound is subjected to an ionization source. Ions from the ionized gas sample are drawn into an ion filter and subjected to a high field asymmetric waveform ion mobility filtering technique. Select ion species allowed through the filter are then passed to an ion detector, enabling indication of a selected species.
The FAIMS filtering technique involves passing ions in a carrier gas through strong electric fields between the filter electrodes. The fields are created by application of an asymmetric period voltage (typically along with a further control bias) to the filter electrodes.
The process achieves a filtering effect by accentuating differences in ion mobility. The asymmetric field alternates between a high and low field strength condition that causes the ions to move in response to the field according to their mobility. Typically the mobility in the high field differs from that of the low field. That mobility difference produces a net displacement of the ions as they travel in the gas flow through the filter. In absence of a compensating bias signal, the ions will hit one of the filter electrodes and will be neutralized. In the presence of a specific bias signal, a particular ion species will be returned toward the center of the flow path and will pass through the filter. The amount of change in mobility in response to the asymmetric field is compound-dependent. This permits separation of ions from each other according to their species, in the presence of an appropriately set bias.
In the past, Mine Safety Appliances Co. (MSA) made an attempt at a functional FAIMS implementation in a cylindrical device, such as disclosed in U.S. Pat. No. 5,420,424. (It is referred to by MSA as a Field Ion Spectrometer (FIS), see FIG. 1.) The device is complex, with many parts, and is somewhat limited in utility.
Fast detection is a sought-after feature of a Wieldable detection device. One characteristic of known FAIMS devices is the relatively slow detection time. However, the GC operates much more rapidly, such that the known FAIMS devices cannot generate a complete spectra of the ions present under each GC peak. Therefore these FAIMS devices would have to be limited to a single compound detection mode if coupled to a GC, with a response time of about 10 seconds. Any additional compound that is desired to be measured will take approximately an additional 10 seconds to measure.
While the foregoing arrangements are adequate for a number of applications, it is still desirable to have a small, fieldable ion detector/spectrometer that can render real-time or near real-time indications of detected chemical compounds, such as for use on a battlefield and in other environments.
Furthermore, a GC-FAIMS arrangement, focused as it is on one species at a time, is incapable of simultaneous detection of a broad range of species, such as would be useful for airport security detectors, or on a battlefield, or in industrial environments. Such equipment is also incapable of simultaneous detection of both positive and negative ions in a gas sample.
It is therefore an object of the present invention to provide a functional, small, fieldable ion detector/spectrometer that overcomes the limitations of the prior art.
It is a further object of the present invention to provide a chemical sensor that features the benefits of GC and FAIMS but is able to operate rapidly with reduced processing time.
It is a further object of the present invention to provide a chemical sensor that features the benefits of GC and FAIMS but is able to detect multiple species at one time.
It is a further object of the present invention to provide a chemical sensor that features the benefits of GC and FAIMS but is able to generate orthogonal data that fully identifies a detected species.
It is a further object of the present invention to provide a chemical sensor that features the benefits of GC and FAIMS but is able to detect positive and negative ions simultaneously.
It is a further object of the present invention to provide a fieldable chemical sensor that includes both a small ion injector/column and a small detector/spectrometer and yet is able to rapidly produce unambiguous orthogonal data for identification of a variety of chemical compounds in a sample.
It is a further object of the present invention to enable a new class of chemical sensors that can rapidly produce unambiguous, real-time or near real-time, in-situ, orthogonal data for identification of a wide range of chemical compounds.
It is a further object of the present invention to provide sensors that have the ability to detect both positive and negative ions simultaneously and achieving reduction of analysis time.
It is a further object of the present invention to provide a class of sensors that have the ability to use the reactant ion peak to extract the retention time data from a GC sample.
It is a further object of the present invention to provide a class of sensors that have the ability to make 2-D and 3-D displays of species information as obtained.
It is a further object of the present invention to provide a class of sensors that enable use of pattern recognition algorithms to extract species information. It is a further object of the present invention to provide a class of sensors that do not require consumables for ionization.
It is a further object of the present invention to provide a class of sensors that provide differential-mobility spectra information in addition to the retention time data.
It is a further object of the present invention to provide a class of sensors that can eliminate the need to run standards through the GC.
It is a further object of the present invention to provide a class of sensors utilizing arrays of FAIMS devices each tuned to detect a particular compound, such that multiple compounds can be simultaneously detected rapidly, with simplified electronics.
It is a further object of the present invention to provide a GC detector which detects compounds by ionizing eluted sample and uses different amplitudes of an applied high filed asymmetric waveform to produce different levels of ion clusters, which can be useful in more precise species identification.
It is a further object of the present invention to provide a class of sensors utilizing arrays of FAIMS devices to provide redundancy in ion detected.
It is a further object of the present invention to provide a class of sensors utilizing arrays of FAIMS devices where each ion filter has its own flow path (or flow channel) and is doped with a different dopant for better compound identification.
It is a further object of the present invention to provide a class of sensors utilizing arrays of FAIMS devices each swept over an assigned bias range of the spectrum to obtain faster analysis of the contents of an eluted GC peak.
It is a further object of the present invention to provide a class of detectors that can provide information on the cluster state of ions and ion kinetics by varying the amplitude of the high voltage asymmetric electric field or by adjusting the flow rate of ions through the device.
It is a further object of the present invention to provide a chemical sensor that features the benefits of GC and FAIMS but is able to detect positive and negative ions simultaneously by providing a longitudinal flow path in which positive and negative ions are carried simultaneously through the filter to the detector for simultaneous independent detection.
It is a further object of the present invention to provide a class of sensors that can detect samples over a wide range of concentrations through a controlled dilution of the amount of sample delivered to the PFAIMS through appropriate control of the ratios the amounts of drift, carrier and sample gasses.
It is further an object of this invention to provide a class of sensors that can quantitatively detect samples over a wide range of concentrations through controlled dilution by regulating the amount of ions injected into the ion filter region by controlling the potentials on deflector electrodes.
These and other objects are well met by the presently disclosed invention. The present invention overcomes cost, size or performance limitations of MS, TOF-IMS, FAIMS, FIS and other prior art devices, in a novel method and apparatus for chemical species discrimination based on differences in ion mobility in a compact, fieldable package.
In one aspect of the invention, a portable chemical sensor is provided. In another aspect of the invention, improvements in laboratory equipment for substance identification are provided. In a preferred embodiment of the invention, a novel planar, high field asymmetric ion mobility spectrometer (PFAIMS) device is coupled with a GC to achieve a new class of chemical sensor, i.e., the GC-PFAIMS chemical sensor.
Embodiments of the present invention enable fieldable chemical sensors that are able to rapidly produce accurate, real-time or near real-time, in-situ, orthogonal data for identification of a wide range of chemical compounds. In one aspect of the invention, a system is provided for generating multiple data for characterizing a chemical species in a gas sample. Sensor systems according to the invention have the capability to render simultaneous detection of a broad range of species, and have the capability of simultaneous detection of both positive and negative ions in a gas sample. With high ionization energy sources, devices in practice of the invention have the ability to use the reactant ion peak to extract retention time data from the GC. They have the ability to generate differential-mobility spectra information in addition to the retention time data and can enable 2-D and 3-D display of species information, and it is even possible to use pattern recognition algorithms to extract species and additional information from the GC-PFAIMS detection data.
Still further surprising is that this can be achieved in a cost-effective, compact, volume-manufacturable package that can operate in the field with low power requirements and yet it is able to generate orthogonal data that can fully identify various detected species.
In practice of the invention, the GC-PFAIMS offers high sensitivity (ppb-ppt) at low cost. These devices can also have the advantage of not requiring any consumables for ionization (like hydrogen gas in a FID). Furthermore, in the field a GC with a flame ionization detector or thermal conductivity sensor must be calibrated using chemical standards, since retention times can shift due to changing environmental conditions (e.g., humidity, moisture etc.). However, in operation of the GC-PFAIMS of the present invention, a different detection principle than that of the GC itself is used, and therefore a second degree of information is provided (i.e., providing differential mobility spectra for each peak of the GC), and this can be used to confirm the experimental results. As such the invention may be used to eliminate the complicated, time-consuming, need to run standards through the GC.
An embodiment of the present invention includes an inlet section, an ionization section, an ion filtering section, an output section for ion species detection, a control section, and a section for gas chromatographic (GC) analysis of a gas sample, the GC section coupled to the inlet section. The ionization section is disposed for ionizing a gas sample from the GC section, the ionized sample passing to an ion filter in the ion filter section. The control section applies a high field asymmetric waveform voltage and a control function to the ion filtering section to control species in the sample that are passed by the ion filter to the output section for detection.
In an embodiment of the invention the ion filter section has at least one substrate and the ion filter includes at least one planar electrode on the substrate, wherein the electrode is isolated from the output section by the substrate.
In an embodiment of the invention, the ion filter section includes a pair of insulated substrates and the ion filter includes at pair of planar electrodes, one on each a substrate.
In an embodiment of the invention, a planar housing defines a flow path between the inlet section and the output section, the housing formed with at least a pair of substrates that extend along the flow path. The ion filter is disposed in the flow path, and the filter includes at least one pair of filter electrodes. At least one electrode is on each substrate across from each other on the flow path. The control section is configured to apply an asymmetric periodic voltage to the ion filter electrodes for controlling the travel of ions through the filter.
In yet another embodiment of the invention, a planar chamber defines a flow path, wherein the GC section separates the gas sample prior to ionization, and filtering proceeds in the planar chamber under influence of the high field asymmetric periodic signals, with detection integrated into the flow path, for producing accurate, real-time, orthogonal data for identification of a chemical species in the sample.
In another embodiment of the invention, the GC further includes a capillary column for delivering the gas sample into the inlet, the gas sample includes a compound-containing carrier gas at a first flow rate. Preferably the inlet section, ionization section, ion filtering section, and output section communicate via a flow path, further including a drift gas source, the drift gas source supplying a drift gas into the inlet to carry the compound-containing carrier gas along the flow path to the output section. One practice further includes a drift gas tube, wherein the capillary column is housed within the drift gas tube, the capillary column having a column outlet delivering the carrier gas and the drift gas flow surrounding the carrier gas flow at the column outlet. One practice including a coupling enabling receipt of the drift gas tube at the inlet with the capillary tube emptying into the inlet section from within the drift gas tube.
In another embodiment, the inlet section, ionization section, ion filtering section, and output section are formed on a planar surface, the planar surface defining a flow path along a longitudinal axis for the flow of ions in a gas sample from the ionization section, through the filter section, to the output section, wherein the output section includes a detector for the detection of multiple ion species simultaneously. Preferably the detector includes a plurality of electrodes for detection of positive and negative ion species simultaneously.
In yet another embodiment of the invention, an ionizer is provided for ionizing the sample and for creating reactant ions, the reactant ions reacting with the ionized sample to create reactant ion data peaks, wherein the control section further includes a circuit for extraction of retention time data from the sample by evaluation of the reactant ion data peaks.
In yet another embodiment, apparatus is provided for generation of complementary data for evaluation of a chemical compound in the sample, that data including retention time and another variable. Preferably the another variable is intensity of the detected ion species.
In practice of an embodiment of the invention, a display is coupled to the output section for display of at least two dimensional data representative of detected species. Preferably the control section further includes pattern recognition part for identification of an ion species according to data detected at the output section. The data includes differential mobility spectra and retention time data in a preferred embodiment.
In yet another embodiment, an isolation part joins the ion filtering section and output section, ions being delivered to the ion filter from the ionization section via a flow path, the isolation part facilitating non-conductive connection of the ion filter and the output section.
In yet another embodiment, the ion filtering section is further characterized by providing a short drift tube for rapid travel of filtered ions to the output part for detection. Preferably the ion filter further includes a pair of electrodes, the electrodes facing each other across the flow drift tube, wherein the ion filter further may include a pair of electrodes, wherein the control section applies the high field asymmetric period voltage and control function as a control field to pair of electrodes to control species in the sample that are passed by the ion filter to the output section for detection, the drift tube defining a first flow path region for application of the control field to ions in the ion filter, the ion filter being located in the first flow path region. The output section further includes an ion detector region, the drift tube defining a second flow path region, the isolation part being located in the second flow path region after the first region and before the detector region, and the ion filter part passes ions in the drift tube under influence of the control field. Ions that are passed by the filter part travel through the isolation part to the detector region for detection, the isolation part isolating the control field from the detector region. Alternatively, further including a pair of substrates, the substrates defining the drift tube, wherein the electrodes are electrically insulated and the substrates are electrically insulating, wherein the substrates may be planar.
In a further embodiment, at least a pair of substrates defines between them a flow path for the flow of ions, with a plurality of electrodes, including a pair of ion filter electrodes, disposed in the flow path between the inlet section and output section, one filter electrode associated with each substrate, the ion filter configured for receiving samples included of a variety of ion species and the filter electrodes cooperating with the control section applying to control the ions, the ion filter simultaneously passing a selected plurality of ion species to the detector part from the sample. Preferably, the output part further includes a detector part, the detector part enabling simultaneous detection of the selected plurality of ion species passed by the filter. The control section may provide separate independent outputs at the detector part, the outputs providing signals representative of species detected simultaneously from within the samples. The detector part may be formed with at least a pair of detector electrodes disposed in the flow path, at least one detector electrode is formed on a substrate, the detector electrodes carrying signals to the independent outputs representative of the detected ion species, one detector electrode being held at a first level and the second detector electrode being held at a second level for simultaneous detection of different ion species passed by the filter.
In an embodiment of the invention, the inlet section, ionization section, ion filtering section, and output section define between them a flow path for the flow of ions, further including a plurality of electrodes, including a pair of ion filter electrodes disposed in the flow path between the inlet section and output section. The plurality of electrodes may include an array of detector electrodes formed in the flow path.
In an embodiment of the invention, the trajectory of an ion passing through the ion filter is regulated by control section, wherein the output section further includes a detector, the detector including a plurality of electrodes in sequence to form a segmented detector, downstream from the ion filter, its segments separated along the flow path to detect ions spatially according to their trajectories.
In an embodiment of the invention, the inlet section, ionization section, ion filtering section, and output section define a flow path, further including a plurality of electrodes defined in the flow path to form an arrangement of electrodes, the plurality defining at least one filter electrode associated with each substrate to form an ion filter section. The system may further include a pair of substrates, wherein the ion filter includes at least a pair of filter electrodes formed on the substrates, the substrates having at least an insulated surface along the flow path located between the filter electrodes and the output section. The system may include a plurality of dedicated flow paths communicating with the output section, wherein the arrangement of electrodes includes an array of filter electrode pairs associated with the dedicated flow paths. The system may include a plurality of dedicated flow paths, wherein the arrangement of electrodes includes an array of detector electrodes in the output part and in communication with the dedicated flow paths. The system may include an arrangement of electrodes includes at least one pair of detector electrodes, one associated with each substrate, wherein the input part further includes an ionization region and further including at least one electrode in the ionization region. The arrangement of electrodes may form a segmented detector with several segments, each segment formed with at least one electrode on a substrate, the segments being formed in a longitudinal sequence along the flow path in the output part. The electronics part may be configured to sweep the applied controlling signals through a predetermined range according to the species being filtered. The substrates may form a device housing, the device housing supporting the input part, flow path, output part, electrodes, and electronics part. A flow pump can be used for drawing a gas sample through the flow path from the input part to the output part. A third substrate may be provided wherein the substrates are planar and define two flow paths. In one practice, the input part includes an ionization source for the ionization of gas samples drawn by the flow pump, further including a second pump for recirculation of air in at least one flow path.
In an embodiment of the invention, a spacer is provided extending along a longitudinal axis defining a flow path between the inlet section and output section and the ion filter disposed in the flow path and including a pair of spaced filter electrodes, the control section including an electrical controller for applying an asymmetric periodic voltage across the ion filter electrodes and for generating a control field, the control field controlling the paths of ions traveling through the filter along the longitudinal axis toward the output section. The spacer can cooperate with the electrodes to form a device housing enclosing the flow path. The outlet may further include a detection area, the spacer defining a flow path extension extending along the longitudinal axis and connecting the input to the detection area, ions passed by the filter traveling to the detection area for detection. The detection area may include at least a pair of detector electrodes, further including an isolation part separating the ion filter from the detector, the isolating part isolating the control field from the detector electrodes. The spacer may further define longitudinal extensions, the flow path extending between the longitudinal extensions and extending along the spacer longitudinal axis. This embodiment may further include a pair of substrates, the substrates cooperating with the spacer for defining the flow path between the inlet and outlet, the substrates further defining the filter electrodes facing each other across the flow path. Preferably the substrates have insulating surfaces that define an electrically insulated flow path portion between the inlet and the outlet, the outlet further including an ion detector. In one alterative, the spacer is silicon and defines confining electrodes in the flow path, further including a detector downstream from the ion filter for detecting ions traveling from the filter under control of the confining electrodes. The outlet may further include a detector, the detector formed with at least a pair of electrodes for detection of ions in the flow path, wherein the controller further defines electronic leads for applying signals to the electrodes. It is further possible wherein the outlet defines an array of detectors, the detectors formed each with a pair of electrodes disposed in the flow path for detection of ion species passed by the filter, or wherein the outlet includes a detector, the detector including a pair of ion detector electrodes, wherein the electronics part is further configured to simultaneously independently enable detection of different ion species, the detected ions being representative of different detected ion species detected simultaneously by the detector, the electronics part including separate output leads from each detector electrode, or wherein the outlet includes a detector having a plurality of electrode segments, the segments separated along the flow path to spatially separate detection of ions according to their trajectories. The ion filter may include an array of filters, each filter including a pair of electrodes in the flow path. Preferably the flow path is planar, further including a source of ions at the inlet, a pump communicating with the flow path for driving of the ions through the filter, and possibly including a heater, in the flow path, for heating the flow path and purging neutralized ions, wherein the heater may include a pair of electrodes, the electrodes having at least one additional function, and the heater electrodes may include the ion filter electrodes. The electrical controller may be configured to selectively apply a current through the filter electrodes to generate heat.
In an embodiment of the invention, a pair of spaced substrates defines between them a flow path between the inlet and an output sections, the ion filter disposed in the path, further including at least a pair of spaced filer electrodes, the filter including at least one of the electrodes on each substrate, the control section further including a heater for heating the flow path. In one practice, the pair of the electrodes on the substrates is used as a heat source for the heater, the control section configured to deliver a heater signal to the heater source. In one practice, a pair of spaced substrates defining between them a flow path between the inlet and an output sections, the ion filter disposed in the path, further including at least a pair of spaced detector electrodes at least one of the detector electrodes on each substrate, the control section further including a heater for heating the flow wherein the control section uses the detector electrodes as a heat source.
In an embodiment of the invention, the control function is a duty cycle control function generated by the control section, a flow path extending between the inlet and output sections, the ion filter disposed in the flow path, the control section selectively adjusting the duty cycle of the asymmetric periodic voltage with the duty cycle control function to enable ion species from the inlet section to be separated, with desired species being passing through the ion filter for detection. In one practice, the asymmetric periodic voltage is not compensated with a bias voltage, further including a detector downstream from the ion filter for detecting ion species that are passed by the filter.
In one embodiment of the invention, a method is provided for generating multiple data for characterizing a chemical species in a gas sample, in a system having a flow path that defines an ion inlet, an output, and an ion mobility filter in the flow path between the inlet and the output, the filter passing ions flowing from the inlet to the output. The methods has the steps of: separating a gas sample with a GC and eluding the separated sample in a carrier gas to the ion inlet, ionizing the sample and applying a drift gas to the sample and carrying the ionized sample to the ion filter, applying an asymmetric periodic voltage to the ion filter for controlling the path of ions in the ionized sample while in the filter, and passing species through the ion filter for detection at the output part. The method may further include the steps of: adjusting the duty cycle of the asymmetric periodic voltage to enable ion species to be separated according to their mobilities, and passing species through the filter according to the duty cycle for detection at the output part.
In another embodiment of the invention, a method is provided for analysis of compounds in chromatography, including the steps of: separating chromatographically a gas mixture to be analyzed in a chromatographic column, ionizing the gas mixture, passing the ionized gas to a field asymmetric ion mobility spectrometer and passing components of the separated mixture through a high field asymmetric ion mobility filter, and detecting ions in the mixture according to their mobilities. The method may further include the step of applying a drift gas to the eluted sample to increase the flow volume and velocity of the ions through the spectrometer. The sample is eluted from the outlet of a capillary column of a GC, and a further step includes surrounding the capillary column outlet with the flowing drift gas. The method also may include the step wherein the system has an ionizer for ionizing the sample and creating reactant ions, the reactant ions reacting with the ionized sample to create reactant ion data peaks, further including the step of obtaining GC retention time by monitoring the fluctuation in intensity of the reactant ion data peaks. Furthermore, the method may include the steps of detecting positive and negative ions simultaneously by passing ions at high RF. The system has an ionizer for ionizing the sample, and a further step includes processing detection data and obtaining retention time, compensation voltage and intensity, and relating this to the sample to identify its species.
In another embodiment of the invention, a sensor system for characterizing a chemical species in a gas sample, includes an inlet section, an ionization section, an ion filtering section, an output section for ion species detection, a control section, and a section for gas chromatographic (GC) analysis of a gas sample, the GC section coupled to the inlet section, and the ionization section disposed for ionizing a gas sample from the GC section, the ionized sample passing to the ion filter section, the control section applying a high field asymmetric period voltage and a control function to the ion filter to control species in the sample that are passed by the filter to the output section for detection, a planar housing defining a flow path between a sample input part and an output part, the housing formed with at least a pair of substrates that extend along the flow path, an ion filter disposed in the flow path, the filter including at least one pair of filter electrodes, at least one on each substrate across from each other on the flow path, and the control section having a control part configured to apply an asymmetric periodic voltage to the ion filter electrodes for controlling the travel of ions through the filter.
The following detailed description is directed to embodiments of methods and apparatus for chromatographic high field asymmetric waveform ion mobility spectrometry for analysis of compounds. It will be appreciated that in practice of the invention, filtering is achieved by accentuating differences in ion mobility. The asymmetric field alternates between a high and low field strength condition which causes the ions to move in response to the field according to their mobility. Typically the mobility in the high field differs from that of the low field. That mobility difference produces a net displacement of the ions as they travel in the gas flow through the filter. In absence of a compensating bias signal, the ions will hit one of the filter electrodes and will be neutralized. In the presence of a specific bias signal, a particular ion species will be returned toward the center of the flow path and will pass through the filter. The amount of change in mobility in response to the asymmetric field is compound-dependent. This permits separation of ions from each other according to their species, in the presence of an appropriately set bias.
It will now be appreciated that in practice of the present invention that the terms detector, spectrometer and sensor have specific meanings. However, these terms also may be used interchangeably from time to time while still remaining within the spirit and scope of the present invention.